I rode the jail trail a few times at about 2am, I was terrified, but in retrospect, these were the most startling moments I've had on it:
-people sitting and talking in benches on the downtown end (seemed odd to me at 2am on a weeknight)
-Bunnies and stray cats crossing your path
-the shock of seeing another bike out at that time
-A (presumably) drunk person sitting on the bench near the eastern end of the trail, was equally as startled to see someone else out at that time as I was
I saw more people talking and sitting and small animals than anything else, just make sure you've got a good light, and be careful not to run anything over.